Construction & Home

Solar Panel Size Calculator

Enter monthly kWh usage, peak sun hours per day, and panel wattage and this solar panel size calculator estimates the system size in kW and the number of panels needed. It is a simplified estimate that ignores system losses.

What this calculator does

The Solar Panel Size Calculator estimates your result instantly from the values you enter, using standard construction & home formulas. Enter monthly kWh usage, peak sun hours per day, and panel wattage and this solar panel size calculator estimates the system size in kW and the number of panels needed. It is a simplified estimate that ignores system losses. Calculations run in your browser, so nothing you type is sent to a server, and the output is an estimate rather than a certified or official figure.

Solar Panel Size Calculator

Result

—
System Size (kW)
—
Panels Needed

About Solar Panel Size Calculator

The Solar Panel Size Calculator estimates a rooftop system size from your monthly kWh usage, the local peak sun hours per day, and the wattage of one panel. It reports the system size in kilowatts and how many panels that requires.

Formula & Mathematical Calculation

System Size (kW) = f(Monthly Usage (kWh), Peak Sun Hours/day, Panel Wattage (W))

Calculated according to standard construction formulas implemented in the calculator core.

Calculation Example

For 900 kWh per month, 4.5 peak sun hours per day, and 400 W panels, the tool computes daily use of 900/30 = 30 kWh, then 30/4.5 = 6.67 kW, and ceil(6.67 x 1000 / 400) = 17 panels. The System Size (kW) box shows 6.67 kW and the Panels Needed box shows 17.

Key Benefits

  • Estimates system size in kW from monthly usage and peak sun hours
  • Divides monthly usage by 30 to get an average daily figure
  • Rounds the panel count up based on your chosen panel wattage

How to Use this Calculator

  1. Enter monthly usage
    Type your Monthly Usage (kWh) from a utility bill.
  2. Enter sun hours and panel wattage
    Type the Peak Sun Hours/day for your area and the Panel Wattage (W).
  3. Calculate
    Click Calculate to read the System Size (kW) and Panels Needed boxes.

Assumptions & Limitations

What the calculation assumes

  • Every value you enter is treated as an exact, known amount.
  • Any rate or interval you enter is assumed to hold for the whole period unless the form asks for it separately.
  • Results use standard published formulas and account only for the values entered above.
  • Rounding is applied for display; the underlying calculation uses your full precision.
  • Results are returned in the same units you enter, unless the tool states the output unit.

What it does not cover

  • Solar Panel Size Calculator produces a calculated result for reference and learning, not a certified or official measurement.
  • Real-world outcomes differ when tolerances, rounding rules or local standards apply.
  • Definitions and standards vary by region and change over time; verify the convention that applies to your case.
  • Figures are only as accurate as the inputs you supply — check them before acting on the result.
  • For work where the result carries real consequences, confirm it against an authoritative source.

Frequently Asked Questions

It divides monthly kWh by 30 for a daily average, then divides by peak sun hours to get the kW size. Panels are that size times 1000, divided by panel wattage, rounded up.

No. It is a simplified sizing that ignores derate factors such as wiring loss, temperature, and inverter efficiency, so real-world systems are often sized larger.

Peak sun hours are the equivalent number of hours per day at full sunlight. Enter a local average; the tool uses it directly to divide the daily energy need.

Free to use, no account needed. Calculations run in your browser, so nothing you type is sent to a server. Results are estimates — how we build and check our calculators.